1. Field of the Invention
The present invention relates to a connection head structure of a high pressure fuel injection tube including a relatively thin-diameter thick-walled steel tube multifariously arranged as a supply path of fuel and the like for, as an example, a diesel internal combustion engine.
2. Description of the Related Art
Conventionally, as a high pressure fuel injection tube having such a connection head, as illustrated in FIG. 5, it is known to comprise a connection head 112 defined, on a connection end portion of a relatively thin-diameter thick-walled steel tube 111, by a spherical seat face 113, an annular flange 115 provided at an interval from the seat face 113 in a tube axial direction, and an arcuate face 114 that tapers toward a tip and continuously extends from the seat face 113 to the annular flange 115 (refer to FIG. 4 of Japanese Published Unexamined Patent Application No. 2003-336560). Such a connection head 112 is formed by working with buckling using pressure of a punch member from the exterior in a tube axial direction, and accordingly, an expansion occurs toward the exterior of a circumferential wall due to the working with buckling using the pressure and thereby creates a pocket (annular recessed portion) 116 on the head inner circumferential face having an enlarged inner diameter and an increased tensile stress of the inner surface due to stress concentration; and the connection head 112 is submitted for use in such a state; but problems such as cavitation erosion occurs proximal to the pocket portion due to the high pressure fluid when arranged and used; and unfortunately radial cracks in the connection head occur in the radial direction due to fatigue rupture with starting points in the pocket, and cracks occur in the circumferential direction due to fatigue rupture around the pocket.
As a countermeasure for such problems, the applicant first proposed, for example, a high pressure fuel injection tube including a connection head defined on a connection end portion of a relatively thin-diameter thick-walled steel tube by a spherical seat face, an annular flange provided at an interval from the seat face in a tube axial direction, and a circular conical surface that tapers toward a tip and continuously extends from the seat face to the annular flange, characterized by a method for making a shallow annular curved groove recess in a portion of the circular conical surface and thereby making the pocket shallow and smooth when formed on the head interior due to formation of the connection head (refer to FIG. 1 of Japanese Published Unexamined Patent Application No. 2003-336560), a method for covering the pocket created on the head interior when forming the connection head wherein an exterior circumferential face defines a truncated circular conical or truncated arcuate seat face for fitting to the opposing seat portion with a metal cylindrical member fitted into the head interior (Japanese Published Unexamined Patent Application No. 2005-180218), and the like.
Further, the applicant proposed a connection head structure of a high pressure fuel injection tube to obtain substantially similar or better effects than the technology first proposed, by proposing a means of preventing the occurrence of cracks in the recessed portion of the pocket due to creation of the pocket when forming the head, the occurrence of cracks due to cavitation erosion occurring proximal to the pocket due to the flow of high pressure fluid when arranged and used, and the phenomena of a diameter enlargement of the inner diameter and an increase of tensile stress of the inner surface due to stress concentration resulting from creation of the pocket when forming the head (Japanese Patent Application No. 2007-61085).
This connection head structure of the high pressure fuel injection tube is characterized in that when a thick-walled thin-diameter steel tube has t (wall thickness)/D (outer diameter)<0.3, a tube axial direction distance L1 from a connection head end to the back face of the annular flange is from 0.38 D to 0.6 D, a spherical body radius R of the seat face is from 0.45 D to 0.65 D, an outer diameter D1 of the annular flange is from 1.2 D to 1.4 D, and the head inner circumferential face is formed as a substantially flat cylindrical face and/or a conical surface having a cross section outline in a tube axial direction which has a diameter nearly the same as that of the inner circumferential face of the steel tube; and when a thick-walled thin-diameter steel tube has t (wall thickness)/D (outer diameter)≧0.3, a tube axial direction distance L1 from a connection head end to the back face of the annular flange is from 0.38 D to 0.7 D, a spherical body radius R of the seat face is from 0.45 D to 0.65 D, an outer diameter D1 of the annular flange is from 1.2 D to 1.4 D, and an inner circumferential face of the connection head is formed as a substantially flat cylindrical face and/or a conical surface having a cross sectional outline in a tube axial direction which has a diameter nearly the same as that of the inner circumferential face of the steel tube; and further for the connection head, an angle θ of an apical angle of a circular conical surface (pressurized seat face) that tapers toward a tip and continuously extends from spherical seat face to the annular flange or to a proximity of the annular flange is from 50 to 60 degrees, a maximum diameter D3 of the circular conical surface is from 1.03 D to 1.09 D, and a circular conical surface, a circular conical surface having a protruding or recessed outline, or a cylindrical face extends from the maximum diameter portion of the circular conical surface to the annular flange.
For the connection head structure of the high pressure fuel injection tube according to Japanese Patent Application No. 2007-61085 recited above, the connection head inner circumferential face has a substantially flat face having a cross section outline in a tube axial direction nearly the same as the inner circumferential face of the steel tube, and therefore almost no pocket (annular recessed portion) is created by plastically working the interior of the connection head; and therefore, excellent effects are provided such as eliminating the occurrence of cracks in the recessed portion of the pocket portion when forming the head, concerns of cracks occurring due to cavitation erosion by fluid pressure inside the head, and phenomena of a diameter enlargement of the inner diameter and an increase of tensile stress of the inner surface due to stress concentration resulting from creation of the pocket when forming the head; drastically reducing the possibility of the connection head inner circumferential face serving as a starting point of fatigue rupture, and the like.
However, it was ascertained that for the connection head structure of the high pressure fuel injection tube according to Japanese Patent Application No. 2007-61085 recited above, an axial direction length of the connection head cannot be sufficiently maintained; and unfortunately, there are concerns that the annular flange may interfere with the opposing part; a risk exists of deterioration of the sealability due to axial misalignment of the nozzle (seal plug) and the tubular body in the case where autofrettage processing is performed on the high pressure fuel injection tube; and in particular, for autofrettage processing with a high pressure of not less than 300 MPa, there are concerns that leaking unfortunately may occur.